The invention is applicable to any type of memory effect electroluminescent display. An electroluminescent display is said to possess a memory effect when the electro-optical property of the display presents a hysteresis loop having two stable operating states. FIG. 1a is a diagram of the hysteresis loop of the electro-optical property of an electroluminescent display exhibiting the memory effect. Display luminance L is plotted up the Y-axis and the electrical voltage V applied to the display is plotted along the X-axis. The more luminous state L.sub.a is called the ON state and the less luminous state L.sub.e is called the OFF state. In order to switch the display from the OFF state to the ON state, the voltage applied to the display is temporarily increased up to a value S.sub.a situated beyond the hysteresis loop. Conversely, the display is switched off simply by temporarily reducing the applied voltage. A "sustain" voltage V.sub.e is permanently applied to the entire display in order to hold all of the pixels in the states they are in.
At present there are two types of memory effect electroluminescent display: there are displays with an inherent memory effect which is obtained when the display includes an electroluminescent layer based, for example, on manganese-activated zinc sulfide interposed between two dielectric layers; and there are extrinsic memory effect displays which are obtained when the display includes a photoconductive layer superposed on an electroluminescent layer.
FIG. 1b shows an example of an electroluminescent display including a photoconductor. The experimental data described below come from a display of this type. However, a display with an inherent memory effect behaves in a similar manner.
An electroluminescent display having a photoconductor comprises a transparent substrate 10, a first set of parallel transparent electrodes or "row" electrodes 12 (with it being assumed that the perspective section shown is taken along one of these rows), an electroluminescent layer 14, a photoconductive layer 16, and a second set of parallel transparent electrodes or "column" electrodes 18 extending perpendicularly to the row electrodes 12.
The row and column electrodes are powered from an A.C. voltage generator 20. More precisely, the row electrodes 12 are connected to the generator 20 via a row driving circuit 22.sub.l and the column electrodes 18 are connected via a column driving circuit 22.sub.c. Observation preferably takes place through the substrate 10 as represented by an eye 23.
The display screen comprises pixels each of which is defined as the overlap zone between a particular row electrode and a particular column electrode. The display per se is, for example, of the PC-EL type, i.e. it is constituted by two layers 14 and 16, one of which is electroluminescent (EL) and the other of which is photoconductive (PC). It will be understood that when the electroluminescent layer is excited, the light which it emits increases the conductivity of the photoconductive layer and consequently increases the conductivity of the PC-EL display itself at the pixel under consideration. Thus, once a pixel has been excited, it can be kept on by applying a sustain voltage thereto, thereby ensuring that said pixel continues to be displayed.
Initially, the driving circuits 22.sub.l and 22.sub.c apply an alternating "sustain" voltage V.sub.e permanently to all of the pixels of the screen, said voltage being taken from the generator 20. More precisely, the row driving circuit 22.sub.l applies a potential U.sub.l to the row electrodes while the column driving circuit 22.sub.c applies a potential U.sub.c to the column electrodes. The voltage across the terminals of each pixel is thus U.sub.l -U.sub.c =V.sub.e. Hereafter, U.sub.c is taken as the reference potential or "ground", giving U.sub.c =0.
The structure of the displayed image is defined and/or modified by the driving function itself.
A conventional mode of driving consists in scanning the row electrodes of the memory display sequentially. Instead of applying the potential U.sub.l, each selected row electrode is subjected to a potential U.sub.la, which is greater than U.sub.l.
Simultaneously, the driving circuit 22.sub.c applies a potential U.sub.ca which is less than U.sub.c to those of the column electrodes crossing the excited row electrode at points where there are pixels to be switched on. It is ensured that U.sub.la -U.sub.ca is greater than a threshold S.sub.a suitable for switching on a previously off pixel, i.e. a pixel at which the photoconductive layer is in a low conductivity state. Thereafter, the alternating sustain voltage V.sub.e is sufficient for keeping switched on those pixels which have been excited in this way.
Conversely, in order to switch a pixel off, a potential U.sub.le which is less than U.sub.l is applied to the selected row electrode and/or a potential U.sub.ce greater than U.sub.c is applied to the corresponding column electrode, with said potentials being applied for a short period of time. The total voltage applied to the corresponding pixels then falls during the short instant of time beneath a second threshold S.sub.e (which is less than S.sub.a) thereby switching off the pixel. The sustain voltage then has no effect on the pixel because of the increased resistance of the photoconductive layer once the pixel has been switched off.
Since the voltages concerned are alternating voltages, the threshold conditions to be satisfied are naturally taken in terms of their peak values.
The switch-off time of memory effect electroluminescent displays is much longer than the switch-on time, with the switch-off time being much greater than one millisecond while the switch-on time is less than 100 microseconds. The optimum method of switching off pixels is therefore different from the above-described switch-on method. For example, it is better to switch off all of the pixels in several rows simultaneously by acting directly on the sustain voltage prior to writing any message. However, in order to simplify the description, it is given in terms of a sequential switch-off method even though the invention is applicable to both methods of switching off.
The various voltage values that need applying to the pixels are associated with currents whose peak values constitute an essential factor concerning the performance and the price of a memory display. The voltage drop along the electrodes due to their resistance must not exceed certain limits, thereby limiting the size of the screen. Also, the cost of the electronic driving circuits is very largely due to the amount of current that they are required to be capable of modulating.
The alternating sustain voltage V.sub.e produces a sustain current through the memory display. This sustain current comprises both a displacement current I.sub.d which is independent of the number of pixels which are switched on, and a conduction current I.sub.c which, in contrast, is proportional to the number of pixels switched on. The driving circuits 22 have both of these currents flowing through them simultaneously. The maximum value of the total current I.sub.t (t)=I.sub.c (t)+I.sub.d (t) is obtained when all of the pixels are switched on.
FIG. 2 is a waveform diagram showing the alternating sustain voltage V.sub.e and the total corresponding sustain current I.sub.t for a single pixel which is assumed to be switched on.
A sustain cycle corresponds to one period of the alternating voltage V.sub.e, i.e. to the time interval lying between instants 0 and T.sub.2, for example. T.sub.2 may be about 1 millisecond, for example.
During a sustain cycle, the alternating voltage V.sub.e reaches its peak value twice, once in the negative half cycle and once in the positive half cycle. In theory, it would therefore be possible to drive two rows sequentially during a single period of the sustain voltage. At a driving rate of 1 kHz, it is therefore conceivable to use a write speed of 2,000 rows per second. However, for practical reasons, some common integrated driving circuits can only switch single polarity voltages. As a result only one of the peaks in the alternating voltage V.sub.e can be used during one period thereof. The maximum writing speed is then only 1,000 rows per second.
Account now needs to be taken of the fact that the row electrodes are generally made of aluminum while the column electrodes are made of indium tin oxide. The row electrodes may also be made of indium tin oxide if it is desired to make a display which is completely transparent. Unfortunately, the resistance of the transparent electrodes 12 made of indium tin oxide is not negligible. However, if reference is made to FIG. 2, it can be seen that while sustaining all of the pixels defined by the overlap zones between the column electrodes and one of the row electrodes, the peak value of the total current I.sub.t is more or less in phase with the sustain voltage V.sub.e when all of the pixels are switched on. Consequently, a voltage drop occurs along the corresponding column electrode 18 and the value of this drop is at a maximum when the sustain voltage V.sub.e reaches its maximum value.
We now consider what happens when the extreme potential values U.sub.la and U.sub.ca are applied to the terminals of the pixel(s) which are to be excited. There is a switching current I.sub.co which corresponds to the row potential increase U.sub.la -U.sub.l and the column potential increase U.sub.c -U.sub.ca. We now restrict ourselves to the currents flowing along the column electrodes during pixel switching, since the column electrodes have higher resistance due to being made of indium tin oxide.
The potential U.sub.c of the column electrodes corresponding to pixels which are not to be switched on is taken as the reference potential and may be equal to 0, for example. However, the column electrodes corresponding to pixels which are to be switched on are taken to a negative potential U.sub.ca for a time T.sub.c. The capacitance per unit area of an off pixel is noted C. The switching current I.sub.co along a column electrode is thus defined by equation (I) (which is to be found in the appendix to the present description together with other equations). This current I.sub.co is added to the sustain current I.sub.c (t)+I.sub.d (t) as defined above. It would therefore be desirable to offset the moments at which the column electrodes and the row electrodes are switched relative to the peaks in the sustain voltage V.sub.e in order to minimize the peak value of the total current flowing through the memory display, thereby reducing the maximum current level that needs to be specified for its circuits. This also serves to limit the voltage drop induced by said total current from one end to the other of the resistive electrodes, and in particular of the column electrodes 18.
From FIG. 2, it appears that there is no way of avoiding having the sustain current overlapping with the switching current I.sub.co : there is no time interval during which pixel switching can take place while the sustain current is zero. The voltage drop from one end to the other of the column electrodes is thus increased during switching by an amount proportional to the peak value of the total current.
It is relatively easy to calculate the relative voltage drop from one end to the other of a resistive electrode when the sustain voltage has a sinusoidal waveform. This voltage drop is expressed in accompanying equation II, where R is the sheet resistivity in ohms/square, L is the length of the electrodes in centimeters, .omega. is the angular frequency of the sustain voltage, C is the capacitance per unit area of a switched-off pixel in nanofarads per square centimeter, and finally k is the factor by which said capacitance is multiplied when the pixel is on.
If it is now assumed that this relative voltage drop must remain less than a maximum A, the maximum height L.sub.M which a display screen can have is given by accompanying equation III. As a result it can be seen that constraints relating to sustaining and/or switching pixels impose a limit on the size of the display screen.
Further, the voltage drop which exists from one end to the other of the resistive electrodes (and in particular the electrodes 18 made of indium tin oxide) has the drawback of producing pixel switch-on and switch-off characteristics which are not uniform over the area of the memory display. This nonuniformity can even give rise to parasitic switching-on or to parasitic switching-off on the matrix screen.
The present invention seeks to provide a solution to this problem.
An aim of the invention is to reduce the peak value of the currents flowing in the column driving circuits and in the column electrodes or in the row and the column electrodes of a memory display.
Another aim of the invention is to increase the speed at which an image can be written.